Methyl-CpG-binding domain protein 1 (MBD1) is a member of a family of mammalian proteins that specifically bind methylated regions of DNA. It is a transcriptional repressor that binds CpG islands in promoters where the DNA is methylated at position 5 of cytosine within CpG dinucleotides. Binding is abolished by the presence of 7-mG that is produced by DNA damage by methylmethanesulfonate (MMS). MBD1 acts as a transcriptional repressor and plays a role in gene silencing by recruiting AFT7IP, which in turn recruits factors such as the histone methyltransferase SETDB1. Isoform 1 and isoform 2 can also repress transcription from unmethylated promoters.